At present, development of an optical wavelength division multiplexing communication system using a plurality of optical wavelengths being actively pursued in order to increase communication capacity. In this optical wavelength division multiplexing communication system, an arrayed waveguide grating type optical multiplexer/demultiplexer circuit is widely used in optical wavelength multiplexing and demultiplexing circuits for multiplexing a plurality of optical signals at the transmitter side or for demultiplexing a plurality of optical signals traveling through one optical fiber into different ports at the receiver side.
FIG. 8 is a block diagram of a conventional arrayed waveguide grating type optical multiplexer/demultiplexer circuit.
As shown in FIG. 8, a conventional arrayed waveguide grating type optical multiplexer/demultiplexer circuit has an input waveguide 1, a first slab waveguide 2 connected to the input waveguide 1, an arrayed waveguide 3 connected to the first slab waveguide 2 and constituted of a plurality of optical waveguides sequentially becoming longer with a predetermined waveguide length differences, a second slab waveguide 4 connected to the arrayed waveguide 3, and a plurality of output waveguides 5 connected to the second slab waveguide 4 (for an example, refer to K. Okamoto, “Fundamentals of Optical Waveguides”, Academic Press, pp. 346-381, 2000). These are constituted by using an optical waveguide which is made up of a core with high refractive index formed on a flat substrate 10 and a clad around the core.
In the case of the conventional arrayed waveguide grating type optical multiplexer/demultiplexer circuit shown in FIG. 8, the light led into the input waveguide 1 is spread in the first slab waveguide 2 and branched to respective optical waveguides in the arrayed waveguide 3. Moreover, the light is multiplexed by the second slab waveguide 4 again and led into the output waveguides 5. In this case, an optical field pattern projected to an end of the first slab waveguide 2 at the arrayed waveguide 3 side is basically replicated to an end of the second slab waveguide 4 at the arrayed waveguide 3 side.
Furthermore, because the arrayed waveguide 3 is provided so that adjacent optical waveguides differ from each other in optical path length by ΔL, an optical field has an inclination which depends on the wavelength of the input light. The optical field changes its focusing position depending on each wavelength by the inclination at the output waveguide 5 side of the second slab waveguide 4 and as a result, it is possible to demultiplex the wavelength. Upon receiving light from the output waveguide 5 side, due to reciprocity of light, light of different wavelength is multiplexed and emitted from the input waveguide 1.
This arrayed waveguide grating type optical multiplexer/demultiplexer circuit is becoming an indispensable optical component in optical multiplex communication systems in that it can make a single optical fiber transmit a plurality of signals of different wavelength.
Moreover, various pass-band-expansion arrayed waveguide grating type optical multiplexer/demultiplexer circuits have been proposed which respectively expand the transmission wavelength band width of the arrayed waveguide grating type optical multiplexer/demultiplexer circuit shown in FIG. 8 (for an example, refer to K. Okamoto and A. Sugita, “Flat spectral response arrayed-Waveguide grating multiplexer with Parabola waveguide horns”, Electronics Letters, Vol. 32, No. 18, pp. 1661-1662, 1996).
FIGS. 9A and 9B are block diagrams of a conventional pass-band-expansion arrayed waveguide grating type optical multiplexer/demultiplexer circuit.
As shown in FIG. 9A, the conventional pass-band-expansion arrayed waveguide grating type optical multiplexer/demultiplexer circuit has a configuration in which a parabola waveguide 6 in a parabolic shape is provided between the input waveguide 1 and the first slab waveguide 2 of the conventional arrayed waveguide grating type optical multiplexer/demultiplexer circuit shown in FIG. 8. As shown in FIG. 9B, when expressing a coefficient as A, the width of the input waveguide 1 as W0, and the length of the parabola waveguide 6 from the first slab waveguide 2 as Z0, the width W of the input optical waveguide 1 contacting with the first 20 slab waveguide 2 is defined with respect to the propagation axis Z of the optical wave by the following equation.Z=A(W2−W02)−Z0
When using this parabola waveguide 6, an optical field formed by the parabola waveguide 6 becomes distributions shown in FIGS. 10A and 10B. FIG. 10A a three-dimensional distribution illustration of an optical field in the parabola waveguide 6 shown in FIG. 9B and FIG. 10B is a two-dimensional distribution illustration of an optical field in the width direction (x direction) of the parabola waveguide 6 at an end portion of the parabola waveguide 6, that is, the boundary between the parabola waveguide 6 and the first slab waveguide 2.
As shown in FIG. 10A, the optical field in the input waveguide 1 has one peak. However, in the parabola waveguide 6 (at the right portion of the position of Z=−Z0 in FIG. 10A), a distribution of optical fields having two peaks is formed. Moreover, the distribution of optical fields at the boundary portion at which the parabola waveguide 6 contacts with the slab waveguide 2 has a double peak as shown in FIG. 10B. Accordingly, at the end of the second slab waveguide 4 at the output waveguide 5 side, the optical field having the double peak is also reproduced and combined with the output waveguide 5. Therefore, it is possible to expand a transmission wavelength band.
However, the conventional pass-band-expansion arrayed waveguide grating type optical multiplexer/demultiplexer circuit provided with the above parabola waveguide has a serious drawback. That is, it has a high wavelength dispersion value due to a phase distribution in the parabola waveguide. FIG. 11 shows graphs of wavelength dispersion and loss with respect to an optical wavelength in the conventional pass-band-expansion arrayed waveguide grating type optical multiplexer/demultiplexer circuit. As obvious in FIG. 11, it is found that the wavelength dispersion value with respect to the optical wavelength is high at the central wavelength, which is taken as a maximum wavelength dispersion value, and greatly changes in wavelength regions around the central wavelength. This wavelength dispersion characteristic causes a problem that an optical signal (pulse) is extremely deteriorated because the dispersion provides different delay times for optical signal spectrum components in one channel.
The present invention is made to solve the above problem and its object is to provide an arrayed waveguide grating type optical multiplexer/demultiplexer circuit with reduced wavelength dispersion.